The development of high-power, low-frequency sound sources and methods of
acoustical tomography has been enhancing the range of observation of underwater
inhomogeneties in the ocean from local to global scale. In this way, the problem
of the role of underwater mountains in sound field formation arises. It also
looks attractive to apply a low-frequency sound for large-scale acoustic ocean
mapping. The characteristic of sound reflection by underwater mountains has not
been studied enough until now. A review of experiments made in the Pacific and
Atlantic on long-range sound reflections from underwater mountains and
continental slopes is presented. During the measurements, continental slopes
were sounded by tone-burst signals of about 240-Hz frequency from distances of
600 and 200 km. Receiving systems deployed at 20--90 km apart from the slopes
detected both the forward and reflected signals. The reflection coefficient was
found to be about -(20--30) dB. This makes it possible to observe the
reflections everywhere in the ocean. Different schemes of long-range ocean
mapping based on low-frequency echo sounding and on coherent and incoherent
aperture synthesis are discussed. [This work was supported by RFBR Grant No.
96-02-1944.]